The long-term objective is to understand how the onset of mitosis is regulated in eukaryotic organisms. These studies use the fission yeast Schizosaccharomyces pombe. Studies of fission yeast have pioneered the discovery of many features of cell cycle control. Most of these controls are conserved amongst a broad range of eukaryotic species, including humans. This conservation is most apparent in the mitotic control, which consists of a group of regulatory proteins that converge to modulate the activity of Cdc2, the cyclin-dependent kinase (Cdk) that binds to cyclin-B and catalyzes mitotic events. Cdc2 is inhibited by phosphorylation on tyrosine-15. This phosphorylation is catalyzed by the kinases Wee1 and Mik1 and reversed by the phosphatase Cdc25. There are three specific aims. (1) Understand how Cdc25 is regulated at the G2-M transition. Activation of Cdc25 by Cdc2 and polo-likes kinases (plks) is proposed to complete a positive feedback loop that is required to initiate mitosis. Genetic and biochemical studies will be performed to evaluate the physiological important of these regulatory mechanisms. (2) Uncover how Wee1 and Mik1 are regulated at the G2-M transition. These studies will test the hypothesis that negative regulation of Wee1 and Mik1 by Cdc2 and Plks is important for mitotic onset. (3) Discover how cytotoxic stress and cell size are connected to mitotic control. A genetic screen as identified new "slm" genes that influence mitosis. These proteins might function down-stream of Spc1 stress-activated protein kinase (SAPK), which links cytotoxic stress to the mitotic control by an unknown mechanism. Alternatively, the Slm proteins might regulate other mitotic control proteins. The slm genes will be cloned and analyzed to uncover their roles in mitotic control. Studies will also be performed to determine if mitotic control proteins undergo nucleocytoplasmic shuttling. The aim of these experiments will be to understand how activation of nuclear Cdc2/cyclin-B is coordinated with signals that indicate cell size and cell physiology that are presumed to emanate from cytoplasm.